Integrated micro fuel processor and flow delivery infrastructure

ABSTRACT

Apparatus for transporting a fluid, atomizers, reactors, integrated fuel processing apparatus, combinations thereof, methods of atomizing reactants, methods of moving fluids, methods of reverse-flow in a reactor, and combinations thereof, are provided. One exemplary apparatus for transporting a fluid, among others, includes: a channel for receiving a fluid; a sensor for determining an internal condition of the fluid in the channel; and a channel actuator in communication with the sensor for changing a cross-sectional area of the channel based on the internal condition, wherein the change in cross-sectional area controls a parameter selected from a pressure and a fluid flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application entitled“Integrated Micro Fuel Processor and Flow Delivery Infrastructure”,filed on Jan. 13, 2004 and assigned Ser. No. 10/756,915, which claimedthe benefit of U.S. Provisional Patent Application Ser. No. 60/440,012,entitled “INTEGRATED MICRO FUEL PROCESSOR FOR HYDROGEN PRODUCTION ANDPORTABLE POWER GENERATION” filed on Jan. 14, 2003, the entirety of whichis hereby incorporated by reference.

FIELD OF THE INVENTION

The present disclosure relates generally to hydrogen production andelectrical current producing apparatus, product, and processes, andrelates more particularly, to fuel generating systems, components usedtherewith, and methods of operating the same.

BACKGROUND

Portable electronic devices, such as handheld computers, laptops, andwireless telephones, are proliferating rapidly for a wide variety ofconsumer, business and military applications. As their use continues toexpand, consumers of all types desire longer power-on times and acontinuously expanding set of functions for them. In order toaccommodate this, a corresponding increase in the demand for portableelectrical power generation and supply needs to be met.

The limited size of these devices places a limit on the size ofbatteries that can be used to power them. This, in turn, places anatural limit on the amount of power conventional batteries can produce.In order to overcome this potential shortfall in portable power for thefuture, other sources of power generation for portable electronicdevices have to be pursued.

Electrochemical fuel cells have a well-recognized potential torevolutionize energy production, for both large-scale and small-scaleapplications. However, as is well known, this potential cannot berealized until simple, cheap and energy efficient means for hydrogenfuel production becomes available. To this end, recent advances inmicro-fabrication have led to the development of compact chemicalmicro-reactors for various small-scale applications, such as on-demandproduction of hydrogen or other chemical fuels useful for portable fuelcell technologies. In addition to the favorable properties of rapid heatand mass transport, the miniaturization of chemical reactors offershigher productivity rates due to the fast, non-equilibrium surfacechemistry properties of the miniaturized reactor.

The above-mentioned size limitations pertinent to portable electronics,coupled with the attractive potential for process intensificationassociated with micro-scale technologies, led to several attempts todesign, fabricate, and test micro-machined chemical reactors forportable hydrogen fuel generation. The most notable examples are thosefrom the Pacific Northwest National Laboratory (PNNL), Motorola EnergyTechnology Labs, Sanyo Corp., Lehigh University, and Innovatek, Inc.These groups each focused on single reaction systems that attempt toconvert known, conventional, large-scale, hydrogen production processesto micro-scale applications. In particular, PNNL has explored catalyticpartial oxidation micro-reactors, whereas the other groups havedeveloped steam-reforming micro-reactors.

A PNNL fuel reformer is depicted in FIG. 1. It has the advantage of asandwich-like design that is complimentary with micro-electro-mechanicalsystems (MEMS) planar (i.e. two-dimensional) fabrication due to easyconnection of sub-systems through common through-holes and structurelamination. However, such a system functions in a sequential series ofstages for mixing, vaporization, combustion, and fuel reforming. Thisstaging results in an increased operating temperature, a higher pressuredrop, a larger reactor size, and requires the use of a complex networkof fluidic channels and heat exchangers. To date, previous efforts havealso failed to overcome certain limitations including operating atreduced reactor “skin” temperatures, which is required for safe portablepower generation.

Accordingly, there is a need for an integrated micro fuel processor forhydrogen production and portable power generation that addresses certainproblems of existing technologies.

SUMMARY

Briefly described, embodiments of this disclosure, among others, includeapparatus for transporting a fluid, atomizers, reactors, integrated fuelprocessing apparatus, combinations thereof, methods of atomizingreactants, methods of moving fluids, methods of reverse-flow in areactor, and combinations thereof.

One exemplary apparatus for transporting a fluid, among others,includes: a channel for receiving a fluid; a sensor for determining aninternal condition of the fluid in the channel; and a channel actuatorin communication with the sensor for changing a cross-sectional area ofthe channel based on the internal condition, wherein the change incross-sectional area controls a parameter selected from a pressure and afluid flow.

An exemplary atomizer, among others, includes: a first reservoir forreceiving a fluid; an atomizer actuator disposed in communication withthe first reservoir for generating an acoustical pressure wave throughthe fluid; and a first set of ejectors including at least one ejectorfor dispensing atomized fluid in response to the acoustical pressurewave.

An exemplary reactor, among others, includes at least one internalchannel for transporting a fluid in a first direction and a seconddirection.

An exemplary integrated fuel processing apparatus, among othersincludes: an atomizer and a reactor fluidically coupled to the atomizer.The atomizer includes a first reservoir for receiving a reactant, anatomizer actuator disposed in communication with the first reservoir forgenerating an acoustical pressure wave through the reactant, and a firstset of ejectors including at least one ejector for dispensing atomizedreactant in response to the acoustical pressure wave. The reactorincludes at least one internal channel for transporting the reactant ina first direction and a second direction to produce a fuel.

An exemplary method of atomizing a reactant, among others, includes:providing an atomizer having at least one ejector nozzle, at least oneatomizer reservoir, and at least one actuator, wherein the atomizerreservoir is disposed between the ejector nozzle and the actuator;activating the actuator to generate an acoustical pressure wave forforcing the reactant through the ejector nozzle; and atomizing thereactant to produce an atomized reactant.

An exemplary of moving a fluid, among others, includes: providing atleast one channel that fluidically couples a first structure to a secondstructure, wherein the channel includes a flexible membrane responsiveto a signal to expand and contract a cross-sectional area of thechannel; and transferring the fluid to the second structure from thefirst structure by causing the flexible membrane to contract thecross-sectional area of the channel while the channel is under aconstant parameter selected from a pressure and a flow rate.

An exemplary of reverse-flow in a reactor, among others, includes:providing a reactor having at least one internal channel fortransporting a reactant in a first direction and a second direction toproduce a fuel, wherein the reactor includes a catalyst disposed on thereactor; introducing the reactant to the reactor in a first direction ata first end of the reactor; and introducing the reactant to the reactorin a second direction at a second end of the reactor along the membrane,wherein introducing the reactant in the first direction and the seconddirection is alternated to achieve a forced unsteady-state operation ofthe reactor, and wherein the reactant reacts with the catalyst toproduce the fuel.

Other apparatus, systems, methods, features, and advantages of thisdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional apparatus, systems, methods, features,and advantages be included within this description, be within the scopeof this disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readilyappreciated upon review of the detailed description of its variousembodiments, described below, when taken in conjunction with theaccompanying drawings.

FIG. 1 is a schematic illustration of a stacked hydrogen fuel reformerof the prior art.

FIG. 2 is an illustration of an integrated system for accomplishinghydrogen production and fuel cell power generation, according to certainembodiments of the present disclosure.

FIG. 3 is an illustration of one embodiment of the micro-atomizer ofFIG. 2 in cross-section.

FIG. 4 is an illustration of various forms of ejectors of themicro-atomizer of FIG. 2.

FIG. 5 is an illustration of a particular embodiment of themicro-atomizer of FIG. 2 that includes an inlet for an external supplyof reagents.

FIG. 6 is an illustration of a stacked micro-atomizer for paralleloperation, according to certain embodiments of the present disclosure.

FIG. 7 is a diagram illustrating the volume change in a reservoir of theatomizer in response to DC voltage signal applied to piezoelectrictransducer.

FIG. 8 is a graph of the simulated peak pressure near the ejectionnozzle in the microatomizer cavity as a function of operating frequencyof a piezoelectric device.

FIG. 9 is a graph of a real part of a simulated pressure response of anejector of the micro-atomizer.

FIG. 10 is an illustration of a pressure response generated by certainchange in the opening size of the microchannel.

FIG. 11 is a side view of a micro-channel having piezoelectric orcapacitive actuation.

FIG. 12 is an illustration of a micro-channel array disposed between twofuel reservoirs.

FIG. 13 is a top view of a micro-channel array according to variousembodiments of the present disclosure.

FIG. 14 is a graphical representation of hydrogen production in amicro-reactor under forward-flow conditions for catalytic oxidation andsteam reforming processes.

FIG. 15 is a graphical representation of hydrogen production in amicro-reactor under reverse-flow conditions for catalytic oxidation andsteam reforming processes.

FIG. 16 is graph displaying quasi-steady-state operating conditions of areverse-flow micro-reactor and optimal locations in its internalchannels for deposition of oxidation and reforming catalysts.

FIG. 17 is an illustration of a planar micro-reactor according tocertain embodiments of the present disclosure.

FIG. 18 is an illustration of a rotating plate micro-reactor accordingto various embodiments of the present disclosure.

FIG. 19 is an illustration of a sliding plate micro-reactor according tovarious embodiments of the present disclosure.

FIGS. 20-22 are graphs showing the effect of the fractal pattern forcatalyst deposition in the channels on enhancement of the local massfluxes and the rates of mass transfer.

FIG. 23 is an illustration of one possible linear (one-dimensional)fractal distribution patterns for catalysts in a micro-reactor.

FIG. 24 is an illustration of an assembled integrated system forhydrogen production, separation, and utilization and the pressuredistribution within the microreactor channels.

FIG. 25 displays an assembled fluid reservoir space and ejector nozzlearray according to certain embodiments of the present disclosure.

FIG. 26 displays a matrix of ejector nozzles according to certainembodiments of the present disclosure.

FIG. 27 displays an etched acoustical horn structure for an ejectornozzle according to certain embodiments of the present disclosure.

FIG. 28 is an illustration of an atomizer directly integrated with amembrane.

FIG. 29 is an illustration of an atomizer integrated with a catalyticmembrane reactor.

FIG. 30 is an illustration of an atomizer integrated directly with afuel cell.

FIG. 31 is an illustration of smart channels integrated with a fuelcell.

FIG. 32 is an illustration of smart channels integrated with a membrane.

FIG. 33 displays silicon micromachined components of the planar reactorwith internal mixing chambers and internal channels according to certainembodiments of the present disclosure.

DETAILED DESCRIPTION

In certain embodiments, the apparatus includes a fuel reservoir incommunication with an actuator-driven, acoustically-responsive,micro-atomizer for providing low-power atomization of gaseous or liquidfuels. The micro-atomizer may include an array of silicon-etched ejectornozzles that are responsive to acoustic waves generated by the actuatorelements to form atomized droplets of the fuel or the like. Themicro-atomizer may also include two arrays of ejector nozzles disposedon opposing ends of the actuator device for providing parallelatomization capabilities.

In further embodiments, the fuel (liquid or gas) is channeled from thefuel tank to the atomizer array or directly to the reactor through amicro-channel or an array of parallel micro-channels, that may bepiezo-electrically and/or capacitively actuated. The channels mayinclude sensors for actuating the piezoelectric device in order toadjust the flow of the reagents there through.

In still further embodiments, the micro-channel delivers appropriateamounts of atomized fuel to a micro-machined, micro-reactor having oneor more channels with an internally deposited catalyst and a hydrogenseparating membrane integrated with the microchannels of the reactor forachieving high efficiency, in-situ hydrogen production. Themicro-reactor is suitable for stand-alone operation as a hydrogengenerator, or in conjunction with an energy-producing fuel cell toproduce electrical power.

In addition to certain known benefits associated with micro-scalereactors in general, enormous opportunities arise to increaseperformance of catalytic processes using forced unsteady-state operation(FUSO) in the catalytic micro-reactor. The FUSO approach, particularlyreverse-flow operation, makes it possible to generate and control thespatio-temporal patterns of temperature, concentrations and catalyststates that are not readily attained under steady-state operation of themicro-reactor. The transient operation provides thermodynamicallyfavorable conditions for reversible reactions, such as decliningtemperature profiles for exothermal reactions and increasing temperatureprofiles for endothermic reactions, so as to achieve maximum reactionconversion and selectivity. FUSO also offers opportunities forexploiting catalyst dynamic properties, and generally results in loweraverage operating temperatures, thereby reducing undesirable pressuredrops and heat losses.

Finally, in still further embodiments, lower-cost fabrication of themicro-reactor can be achieved by using fractal distribution of thecatalyst over its internal channels, rather than a continuous depositionof catalyst, without unduly impacting reactor efficiency.

Turning now to FIGS. 2-33, wherein similar components of the presentdisclosure are referenced in like manner, various embodiments andcomponents of an integrated micro fuel processor for hydrogen productionand portable power generation are disclosed.

1. Integrated Fuel Processing Generator

FIG. 2 displays a schematic of an embodiment of an integrated system 100for accomplishing hydrogen production and power generation via a microfuel cell. The system 100 includes: a fuel reservoir 200 for storing afuel; an array of channels (e.g., smart microchannels 300) fordistribution of fuel between reservoir and atomizer; anactuation-driven, micro-machined, acoustic atomizer 500 for low-powerand on-demand atomization of fuels received from the fuel reservoir 200via the channels; mixing chambers 400 for mixing the atomized fuelreceived from the atomizer 500; and a micro-machined catalyticmicro-reactor 600, which is operated either in a unidirectional or areverse-flow mode. The micro-reactor 600 includes one or more internalchannels having a catalyst deposited therein for producing reactionproducts and may or may not have a hydrogen separating membrane 700 forhigh efficiency, in-situ hydrogen production therefrom. Components200-700 are suitable for stand-alone operation as a hydrogen generator,or may, in certain embodiments, be operated in conjunction with a fuelcell 800 (e.g., a hydrogen fuel cell). In addition, the individualcomponents (individually or in various combinations) may be used forapplications other than fuel processing.

The integrated system 100 disclosed herein has the following generalfeatures in comparison to existing large-scale and sequentialmicro-scale systems. The micro-atomizer 500 may include an array ofactuation-driven (e.g., piezo-electrically driven) ultrasonic ejectors(described in detail below with respect to FIGS. 3-7) that may beintegrated with mixing chambers 400 for highly efficient atomization,evaporation, and mixing of the liquid reagents/fuel prior to theirintroduction to the micro-reactor 600.

The array of smart micro-channels 300 may connect the fuel reservoir tomixing chambers and the fuel atomizer described in FIGS. 10-13. Themicro-channels 300 may also include pressure or other flow rate sensorsdisposed in a feedback loop to an actuator (e.g., a piezoelectric orcapacitive actuator) that allows for precise control of the channelopening area and therefore pressure or other internal conditions of thefuel reservoir 200, the mixer 400 and/or the micro-atomizer 500, inorder to optimize the total fuel throughput to the micro-reactor 600.

The micro-reactor 600 exploits the small size of its several internalchannels, which may each be disposed with catalyst on an internalsurface thereof in order to facilitate reaction kinetics. The chemicalprocessing rates in a heterogeneous micro-reactor 600 increasesignificantly due to a decrease in the resistance to the speciestransport that is caused by a drastic reduction in the thickness of theboundary layer of the channel flow. Ideally, with a decrease in thecross-sectional area of its internal channels, reaction kinetics of themicro-reactor 600 can be achieved at an intrinsic rate and asufficiently large reactor throughput can be readily maintained by usingmany, parallel, internal channels.

The high pressure differential needed to pump reagents through themicro-reactor 600, a potential disadvantage in prior systems, is insteadexploited to provide sufficient pressure differential for hydrogenseparation across the hydrogen separating membrane 700. Since pressuredrop is inversely proportional to the dimension of the channel to thefourth power, an appropriate pressure difference across the membrane 700can be readily established by varying the size of the internal reactorchannels, the thickness of the membrane 700, and the back-pressure inany hydrogen-collecting manifold on the opposite side of the membrane700. This essentially eliminates the need for an additional pump for theseparation stage, as may be found in prior sequential hydrogenproduction systems.

Since the temperature inside a chamber of the micro-reactor 600 (but notof the reactor skin) may be high, particularly in the case ofauto-thermal catalytic conversion of the hydrocarbon fuels, the hydrogenseparation membrane 700 will be maintained at a sufficiently hightemperature (about 400-600° C.) to achieve high permeability of hydrogenfor a given pressure differential. This eliminates the need foradditional heating equipment, such as burners and/or heat exchangersused for membrane heating during the separation stage in conventionalsystems.

Lastly with respect to the micro-reactor 600, the proposed reactorconfiguration is well suited for scale-up by stacking identical planarreactor units on top of each other to achieve increased hydrogen outputfor greater power applications.

The integrated system 100 may facilitate direct hydrogen removal fromthe micro-reactor 600, without waiting for the reaction products toleave the micro-reactor to start separation. This shifts the equilibriumof reversible steam reforming and water-gas shift reactions towards muchincreased rate of hydrogen production. Once removed from the reactionstream, hydrogen can either be collected in a storage manifold or, moreoptimally, be directly delivered to an anode of the fuel cell 800 thatmay be integrated with the micro-reactor 600.

Each of the several components 200-800 will be described in turn andparticular functions of the integrated system 100 will now be described.The choice and design of the components 200-800 described herein willfirst depend on the types of fluid or fuel being supplied to operate theintegrated system 100. Ideally, gaseous hydrogen could be supplieddirectly to the fuel cell 800. This would forgo the need for includingthe micro-reactor 600 and the hydrogen separator 700 described herein.Such an embodiment of the system 100 could then utilize the followingglobal electrochemical reaction at the fuel cell 800 to generateelectricity:2H₂+O₂→2H₂O+electricityThis embodiment of the system 100 would be advantageous in that therewould be no need for hydrogen producing devices such as components 600and 700, and would readily generate a high power density. However, cheapsources of hydrogen fuel or hydrogen storage means are not readilyavailable. Design of a system to withstand such conditions would behighly difficult and complex.

One alternate fuel choice, on which various of the following embodimentsof the disclosure are based, is methanol or methane or any otherhydrocarbon-based fuel. This alternate embodiment of the integratedsystem 100 may include the micro-reactor 600 and the hydrogen separator700 for accomplishing hydrogen production. Such an alternate embodimentmay utilize the chemical reactions described later below with respect toFIGS. 14-16 to produce hydrogen, in particular. Although for sake ofbrevity, the embodiments herein are described particularly with respectto methanol fuel and hydrogen production, it is readily contemplatedthat the integrated system 100 may be adapted to use wide variety ofother chemical reactions, liquids, and fuels to produce a variety ofreaction products.

In accordance with the previous descriptions, the integrated system 100may first include a fuel reservoir 200 for storing liquids or fuels,such as a methanol fuel. The fuel reservoir 200, in certain embodiments,may be an external fuel supply, and may be a permanently-affixed,refillable and/or disposable container of such fuel.

In additional embodiments, the fuel reservoir 200 may be internal to themicro-atomizer 500. In such embodiments, the fuel reservoir 200 and/orthe micro-atomizer 500 may include an inlet for receiving the fuel froman external source and temporarily storing the fuel prior to its beingatomized and dispensed from the micro-atomizer 500.

2. Smart Channels

Referring now to FIGS. 10-13, and continuing reference to FIG. 2, thesmart micro-channels 300 will now be described. Micro-channels 300, andarrays of such micro-channels 300, may be used to control the transportof fluid (gas or liquid) to be atomized through fluid channels 306within the integrated system 100, or may be used for other applications.A fluid channel 306 may be micro-machined on any substrate material,preferably silicon, that is covered on at least one side (e.g., a topsurface) or opposing sides by flexible membranes (e.g., comprisingdeposited dielectrics, such as silicon nitride or elastomers)

The micro-channels (300) may include a piezo-electric actuator 310 thatdisplaces the flexible membrane, thereby causing the cross-sectionalarea (opening) of each fluid channel 306 to be increased or decreased,(FIGS. 10, 320, 330 and FIG. 11) at one or several locations along eachmicro-channel 300. In another embodiment, the actuator may be of acapacitive action which employs a top electrode 303 and a bottomelectrode 305 to form a capacitance, as shown in FIG. 11.

When the cross-sectional area of the fluid channel 306 decreases(increases), the hydraulic resistance to flow increases (decreases)drastically and, thus, either the flow rate of the fluid decreases(increases) drastically if the pressure difference is maintainedconstant between the two fluid reservoirs or the pressure differencebetween the fluid reservoirs increases (decreases) drastically if theflow rate through the channel is maintained constant. Application of DCand AC signals to the desired actuator electrodes at different locationsof the channel can be used to expand/contract the channel with a desiredspatial and/or temporal pattern, such as providing a peristaltic pumpaction. As a result, for example, the sequential expansion andcontraction of the channel along its length may also generate thepumping action to transport fluid from one reservoir (e.g., fuel tank)to another reservoir (e.g., atomizer or mixing chamber of themicroreactor).

In a particular example, a micro-channel array may be used to maintainpressure at the desired level (e.g., constant) in one of the fuelreservoirs 200, or in the chambers that lead to the mixer 400 or to themicroatomizer 500 or to any other liquid delivery/storage system 200,FIG. 10 that are connected by the micro-channel 300.

Similar structures can be used in other applications where precisecontrol and pumping of fluid or gas flow is needed, especially at lowfluid or gas flow rates. The smart microchannel array (see FIG. 13)consists of the following two main components: (1) the array of channelsmicromachined on any substrate material (preferably silicon) that arecovered on one side (top, as shown in FIG. 11) or both sides by flexiblemembranes (e.g., deposited dielectrics such as silicon nitride orelastomers); and (2) each channel is supplied with one or severalactuators of the piezoelectric or capacitive type.

Each actuator can individually be activated and controlled by supplyingproper combination of DC bias and AC voltage signals to thepiezoelectric material or electrodes of the capacitive actuator 302, asshown in FIG. 13.

Since a small change in the size results in tremendous change in theflow rate that is proportional to the forth power of a change in thechannel hydraulic diameter for the constant pressure drop between twoconnected fluid reservoirs 200 and 400, as shown in FIG. 12, andlikewise, since the volumetric flow rate increases proportionally to theforth power of an increase in the hydraulic diameter of themicro-channel 306 for the constant pressure drop between the two fluidreservoirs 200 and 400, a high signal amplification factor (orsensitivity) can be achieved using the above configurations for themicro-channel 300. Likewise, a high dynamic range of operation is madepossible because the total flow rate or pressure difference between thetwo fluid reservoirs 200 and 400 may be controlled through very precisecontrol of the resistance to the flow in each individual micro-channel300. On-demand switching and easy integration with electronics is alsoreadily accommodated. For example, the ability to controllably changeon-demand the cross-sectional area of the channels provides thefoundation for optimal design of the flow cells in chemical sensors asdescribed in Phillips, C., Jakusch, M., Steiner, H., Mizaikoff, B., andFedorov, A., 2003, “Model-Based Optimal Design of Polymer CoatedChemical Sensors”, Analytical Chemistry, Vol. 75, No. 5, pp. 1106-1115,which is incorporated herein by reference.

FIG. 13 displays a top view of a particular embodiment including severalmicro-channels 300 forming a micro-channel array 340. Each micro-channel300 therein may include an inlet 312 for receiving a liquid or a fuelfrom an external component. Each micro-channel 300 of the array 340 mayfurther be actuated by an individually addressable piezoelectric orcapacitive actuators 310, 303, and 305.

3. Microatomizer

Turning now to FIGS. 3-7, therein are depicted various embodiments ofthe micro-atomizer 500 that may be used alone for a variety offunctions, or may be part of integrated system 100. Efficientatomization and mixing of liquid fuels is of paramount importance forcompact and energy efficient fuel evaporation prior to hydrogengeneration. However, fluid atomization has many other applications,including: drug delivery, encapsulation and scaffolding forpharmaceuticals; mixing and drug delivery via inhalation;chip-integrated cooling of electronic components; ink jet printing;providing patterned deposition of photoresist and coatings; ionizationof compounds (molecules) for mass spectroscopy; low temperature &pressure, controllable generation of monodisperse droplets; and reagentdelivery for high throughput drug screening essays. Subsequently,depending on the application, the fluid that is ejected can include, butis not limited to, a polymer, a suspension with solid particles orproteins, a gas, liquid (e.g., pure liquid or a liquid mixture withparticles), and combinations thereof. The micro-atomizer 500 describedherein is suited for all of these tasks, as well as performing similarfunctions in other chemical/reactor engineering, pharmaceutical,biomedical, thermal management, semiconductor manufacturing, materialscience, biology and analytical chemistry applications.

FIG. 3 displays a cross-section of one embodiment of the micro-atomizer500, which has one or more ejectors 535 that uses resonant, focusedacoustic waves 515 in liquid or gas acoustic horns to generate on demandhigh pressure gradients near micromachined nozzles 540 resulting incapability for controllable generation of monodisperse droplets at lowtemperature and pressure. The nozzle 540 forms an opening through whichatomized fuel may be dispensed. The acoustic waves 515 may be generatedby an integrated piezoelectric device 510, described in detail furtherbelow. In certain embodiments, the piezoelectric device 510 can bereplaced by a micromachined capacitive ultrasonic transducer asdescribed in several references (X. C. Jin, I. Ladabaum, F. L.Degertekin, S. Calmes and B. T. Khuri-Yakub, “Fabrication andCharacterization of Surface Micromachined Capacitive UltrasonicImmersion Transducers”, IEEE/ASME Journal of MicroelectromechanicalSystems, 8, pp. 100-114, 1999, which is incorporated herein byreference).

In certain embodiments, the ejectors 535 and the integratedpiezoelectric device 510 should have to include an internal reservoir550 for temporarily storing the fuel or liquid 525 and creating themedia/conditions for waves to propagate prior to atomized dispersal ofthe same. For example, the dimensions of the internal reservoir and thefocusing acoustic horn can be selected to excite a cavity resonance inthe atomizer at a desired frequency. The structures may have cavityresonances in about the 100 kHz to 100 MHz range, depending on fluidtype, dimensions and cavity shape when excited by the piezoelectricdevice 510.

The structure for the ejectors 535 may be etched from silicon 530 orother materials that has substantially higher acoustic impedance ascompared to the fluid filling the atomizer and the reservoir to makesure that the structure can confine and focus acoustic waves, asdescribed later below. The structure of the ejectors 535 may, in variousembodiments, be conical, pyramidal or horn-shaped with different crosssections, and may have a tri-angular cross-section as shown in FIG. 3.The structure of the ejectors 535 may have any cross-sectional area thatis decreasing (linear, exponential or some other functional form) from abase of the structure to the nozzle 540. Such structures may haveacoustic wave focusing properties in order to establish ahighly-localized, pressure maximum substantially close to the nozzle540. This results in a large pressure gradient at the nozzle since thereis effectively an acoustic pressure release surface at the nozzle. Sincethe acoustic velocity is related to the pressure gradient throughEuler's relation, a significant momentum is transferred to the fluidvolume close to the nozzle during each cycle of the acoustic wave in theejector cavity. When the energy coupled by the acoustic wave in thefluid volume is substantially larger than the restoring energy due tosurface tension and other sources, the fluid surface is raised from itsequilibrium position. Furthermore, the frequency of the waves should besuch that there is enough time for the droplet to break away from thesurface due to instabilities.

The nozzle size effectively determines the droplet size and the amountof pressure focusing along with the cavity geometry. The nozzle can beformed using various micromachining techniques as described below andcan be circular, rectangular and other shapes. One further considerationwith the size of the nozzle is that it is desirable, although notnecessary to have the nozzle radius to be in the order of the wavelengthof the capillary waves at the operation frequency. This insures that theenergy of the acoustic waves results in droplet ejection as opposed tosimply transferring energy to the capillary waves to oscillate the fluidsurface.

The ejectors 535 may include one ejector 535, a (one-dimensional) arrayof ejectors 535 or a (two dimensional) matrix of parallel arrays ofejectors 535. As shown in FIG. 4, the ejectors 535 may include onenozzle 540 each or include a plurality of nozzles 540 in a singleejector 535. In certain embodiments of ejectors 535 integrated with thepiezoelectric device 510, the micro-atomizer 500 may include a secondejector or array or matrix of ejectors 535 disposed on opposing sides ofthe piezoelectric device 510 to provide parallel atomization of theliquid or fuel 525, as shown in FIG. 6.

The design of the atomizer is what allows for the low operatingtemperature and pressure, as well as the controllable generation ofmonodisperse droplets of the fuel or liquid 525. Integration of theejectors 535 with the piezoelectric device 510 readily accommodateson-demand switching, manipulation and control of both flow rates andpressure in order to achieve highly uniform atomization at low flowrates in the micro-atomizer 500, or in other components in communicationwith the micro-atomizer 500. The integrated micro-atomizer 500 alsoprovides control of the velocity and uniform diameter of ejecteddroplets, which may range from tenths of micrometers to severalmicrometers (μm), depending on the size of the nozzles 540. Theintegrated micro-atomizer 500 also allows for low temperature operationand low power consumption due to resonant operation. A sensor (notshown) may be readily included with the micro-atomizer 500 for detectinga condition, such as internal pressure, within the internal fuelreservoir 550 and automatically actuate the piezoelectric device 510 tocorrect the detected internal condition during operation of themicro-atomizer 500.

EXAMPLE 1

The following illustrates a method for manufacturing an assembly of themicroatomizer. This is not the only method for fabrication:

Low-cost manufacturing and assembly of the micro-atomizer may berealized using the following MEMS batch fabrication process. First, asilicon wafer is coated on opposing sides with a silicon nitride maskinglayer (for example, Si₃N₄). Next, photolithography and inductivelycoupled plasma (ICP)-etch techniques may be used to define the areas onone side of the silicon wafer for the ejector(s) of the micro-atomizer500. A potassium hydroxide (KOH) anisotropic etch of silicon is thenperformed to define the pyramidal shaped horn structure of the ejectors535 and the nozzles 540. The nitride layer is then removed usingreactive ion etching (RIE) process. If necessary, the nozzles 540 maynext be opened from the back-side of the silicon wafer using a KOH etchor an ICP etch. The ejectors 535 may then be cut from the wafer, using adicing saw or other similar device, after which the array of ejectors535 may be prepared for fabrication of the internal fluid reservoir 550.

MEMS fabrication of the internal reservoir 550 of the micro-atomizer 500may proceed in a similar manner. First, a silicon wafer is coated withsilicon nitride. Lithographic techniques are then used to define thearea of the internal reservoir 550 and an inlet 504 from an externalsupply, if desired. Any necessary through-holes for the internalreservoir 550 may then be KOH- or ICP-etched. The structure for thefluid reservoir is then diced or cut from the wafer.

The structure of internal reservoir 550 is then prepared and bonded oradhered to the ejector 535, with the nozzles 540 pointed away from theinternal reservoir 550. The piezoelectric device 510 (such as atransducer, a piezoelectric single crystal or a piezoelectric ceramic)is cut and cleaned. Electrodes, such as platinum or silver electrodesare then deposited on the piezoelectric device 510. Electrical leads 502may then be bonded to the electrodes in order to provide alternatingcurrent and/or direct current signals to actuate the piezoelectricdevice 510. Finally, the piezoelectric device 510 is adhered or bondedto the structure of the internal reservoir 550 on an opposite side fromthe ejectors 535 to form the micro-atomizer 500, shown in cross-sectionin FIG. 5.

A micro-atomizer 500 formed in any of the foregoing manners may usemegahertz (MHz) frequency acoustic waves to resonate theacoustically-responsive ejectors 535 and potentially to amplify theacoustic energy at the free surface of liquid at the nozzle tip, andthus facilitate atomization with low power consumption. It should beclear that higher (lower) frequency may be used as long as attenuationin the fluid or losses are not excessive. In one embodiment, a singlepiezoelectric device 510 may be used to drive an entire array or matrixof injectors 535, which is kept cool by continual immersion in theliquid fuel. The size of ejected droplets 560, droplet ejection speedand the flow rate may be controlled by the size of the nozzle 540, theamplitude and frequency of acoustic waves 515, respectively, generatedby the piezoelectric device 510.

3.1 Atomizer with Pressure Control of Liquid in the Fuel Reservoir.

For controllable and stable fuel atomization, the pressure of the fuelin the reservoir 550 needs to be maintained at a certain level. Thepiezoelectric transducer 510 can be used for that purpose in addition toproviding the AC driving for atomization. This is achieved by applying aDC bias voltage in addition to the AC atomization signal as shown inFIG. 7. The volume change induced by the piezoelectric transducer can beenhanced by forming a bimorph structure or a membrane structure, ifneeded. For example, initially, the piezoelectric can be biased toincrease the reservoir volume, applying suction to the liquid fuelstored in an external fuel container. During operation, the bias can bechanged to move the transducer up reducing the volume of the reservoir.A one way valve (not shown) can be used to prevent the return of thefuel sucked in during the initial fuel reservoir loading to the externalfuel container. In this case the valve operation is similar to anelectrical diode where the flow is analogous to electrical current. Thepressure can be controlled in a closed loop using a pressure sensor inthe fuel reservoir generating the drive signal for the piezoelectrictransducer.

In order to increase the throughput of chemical processing in the system100, the ejector arrays 535 can be stacked vertically so that a singlepiezoelectric device 510 can drive the atomization from more than oneinternal fuel reservoir 550 to generate atomized fuel droplets 560, asshown in FIG. 6.

Although particular embodiments of the ejectors 535 have been describedabove, it is contemplated that a variety of other embodiments may beemployed. Instead of being formed from silicon, the ejectors 535 mayinstead be etched, stamped or otherwise formed from a variety ofmaterials, including aluminum, other metals, and plastics.

EXAMPLE 2 Modeling and Performance of a Silicon Microatomizer

Experimental results from a modeling of the silicon micro-atomizer 500are shown in FIGS. 8-9. Modeling of the acoustic response of themicro-atomizer 500 was performed using ANSYS finite element calculationsoftware to identify the resonant frequencies of the micro-atomizerstructure (including the ejector 535, the internal reservoir 550 and thepiezoelectric device 510) that would result in the greatestamplification of the pressure gradient at the nozzle 540. The simulatedmicro-atomizer 500 consisted of a number of identical ejectors etched insilicon. A one millimeter (mm) thick piezoelectric device 510 wassupplied with a sinusoidal voltage signal, which generated an acousticwave 515 into the liquid-filled ejector 535. In this example, thepiezoelectric device is made of PZT-5H, a common piezoelectric ceramic.Multiple reflections of acoustic waves 515 from the walls of the ejector535 and the internal reservoir 550 established a standing acoustic wavein the ejector 535 with the maximum pressure gradient at the nozzle tip540, from which the atomized fuel droplet 550 is dispensed.

In simulations, a one-dimensional (1-D) triangular ejector 535 havinginternal wall angles of 54.74°, which would result from a conventionalanisotropic KOH etch of a silicon wafer, was filled with water andacoustic waves were produced by a piezoelectric device 510 actuated witha 1.0 volt signal at different frequencies. The goal of the simulationswas to identify the resonant frequencies of the ejector 535 that wouldresult in the greatest amplification of the pressure at the nozzle 540.Simulation of the frequency response of the ejectors 535 were performedfor piezoelectric device 510 driving frequencies ranging from 1 to 6 MHzto find out the frequency or frequencies at which resonance occurs inthe ejector 535. The maximum pressure amplitude at a node near the tipor nozzle 540 of the ejector 535 is displayed in FIG. 8. It can beobserved that the first resonance occurs around 1.7 MHz, and the secondresonance occurs at around 2.47 MHz frequency, which are readilyachievable with the available piezoelectric materials, for examplePZT-5H or PZT-8 bulk piezoelectric ceramics.

A contour plot of the real part of the pressure amplitude at the secondresonance frequency (2.47 MHz) is shown in FIG. 9, where a nozzle widthof 20 μm is assumed. It can be seen that acoustic waves 515 indeed focusclose to the nozzle 540 where the pressure amplitude is increasedsignificantly near the nozzle tip establishing sufficient pressuregradient for droplet ejection. When the nozzle 540 is open, a pressurerelease surface will be introduced resulting in a larger pressuregradient around the nozzle, thus forcing the formation of atomizeddroplets 550. Note that the simulations are done using a 1-D model forthe ejector, so the focusing occurs only in one dimension. With a 2-Datomizer structure the focusing will be enhanced due to concentration ofthe energy by the reduced area.

To validate the simulations of the micro-atomizer 500, a device wasconstructed according to the parameters listed above for the examples 1and 2 and a sinusoidal voltage signal was applied to a bulkpiezoelectric transducer 510 with the frequency gradually varied fromone to three Megahertz. The first resonance was observed at both 1.481MHz, and the second one was found at 2.263 MHz, both of which correspondwell with theoretically predicted values. At these frequencies, theresonant pressure amplification occurs at the nozzle 540 and severalthin jets of water droplets were emitted from the ejector 535. On-demandturn on-off switching was achieved by changing the amplitude of thevoltage signal applied to the piezoelectric device 510.

3.3 Spatially and Temporally Controlled Droplet Ejection and Control ofEjection Parameters

In additional embodiments, the ejectors 535 may have one or severalindividually-addressable nozzles 540. This may be accomplished in any ofa variety of manners. In particular, the piezo-electric device 510, orthe several electrodes 570 thereof, may be partitioned, segmented orotherwise individually arranged to excite one or several particularejectors 535 upon actuation. Alternatively, or in addition thereto, anelectrostatic, piezoelectric or magnetic on/off actuator (not shown) maybe disposed at the nozzle 540 to, for example, open and close the nozzle540 in an individually addressable manner while the piezoelectric device510 is activated with a continuous wave signal. In this manner, thespatial and temporal distribution of the droplets can be controlled.

The microatomizer can also be used for drop-on-demand operation in timedomain by modulation of the actuation signal. The piezoelectric device510 can be excited by a finite duration signal with a number ofsinusoidal cycles (a tone burst) at the desired frequency. Since acertain energy level needs to be reached for droplet ejection, duringthe initial cycles of this signal the standing acoustic wave pattern inthe resonant cavity is established and the energy level is brought up tothe ejection threshold. The number of cycles required to achieve thethreshold depends on the amplitude of the signal input to piezoelectricdevice 510 and the quality factor of the cavity resonance. After thethreshold is reached, one or more droplets can be ejected in acontrolled manner by reducing the input signal amplitude after thedesired number cycles. This signal can be used repetitively, to eject alarge number of droplets. Another useful feature of this operation is toreduce the thermal effects of the ejection on the piezoelectric device510, since the device can cool off during when the ejector is turned offbetween consecutive ejections. The ejection speed and droplet size canalso be controlled by the amplitude of the input signal and duration.These parameters can be optimized for particular microatomizerapplication.

4.0 Micro Reactor

4.1 Principles and Benefits of Reverse-Flow Autothermal Operation:Background

Turning now to FIGS. 14-16, a micro-reactor 600 and varioussub-components thereof will now be described. Micro-scale fabrication ofthe reverse-flow micro-reactor 600 provides several attractive designfeatures in comparison to large-scale hydrogen production systems. Inparticular, it removes diffusion limitations (˜1/Diameter) on thekinetics of chemical reactions, thereby allowing them to run at theirintrinsic rate while maintaining sufficient reactor throughput by usingparallel chemical processing in many identical channels. There is anincrease in the reaction productivity that is achievable by exploiting anon-equilibrium, ultra-fast surface chemistry of the interior of themicro-reactor 600 that drastically decreases the time constant(1/Diameter²) for internal flow of reactants. Finally, micro-scalefabrication techniques offer unmatched opportunities for precise controlof surface properties and placement of catalyst at optimal locations inthe internal channels of the micro-reactor 600.

In order to discuss a particular design of the hydrogen producingmicro-reactor 600, it is first necessary to determine a preferredchemical reaction from which hydrogen production will take place. Tothat end, and given the inherent size limitations of the system 100,catalytic reactions have been determined to be the most readilyaccommodated by the micro-reactor 600.

The known chemical processes for producing hydrogen from a generichydrocarbon fuel C_(n)H_(m) are as follows:Partial Oxidation: C_(n)H_(m) +nO₂ →nCO₂+(m/2)H₂ (slightly exothermic)Full Oxidation: C_(n)H_(m)+(n+m/4)O₂ →nCO₂+(m/2)H₂O (stronglyexothermic)Steam Reforming: C_(n)H_(m) +nH₂O→nCO+(n+m/2)H₂ (strongly endothermic)Water-Gas Shift: CO+H₂O→CO₂+H₂ (slightly exothermic)

Among these various alternatives, production of the hydrogen-richsynthesis gas (H₂/CO mixture with some CO₂, H₂O, and N₂ reactionproducts) followed by the hydrogen separation from the stream viacomposite metal (palladium-copper alloy or palladium-silver) is one ofthe most economically plausible processes for hydrogen production.Conversion of various hydrocarbon fuels (e.g., methane or methanol) intoa synthesis gas can be accomplished by a variety of differenttechniques. Two main forms of catalytic reactions for producing hydrogengas using methane fuel as a reactant are identified herein, namely,partial oxidation and steam reforming.

A micro-reactor utilizing catalytic partial oxidation demonstrates afaster start-up time, respond more rapidly to change in fuel feed, andbe more compact than with other processes. However, catalytic oxidationproduces a lower hydrogen yield and must operate at higher internaltemperatures than other available processes. On the other hand,catalytic steam reforming produces a higher hydrogen yield at lowermicro-reactor temperatures as compared to catalytic oxidation processes.However, this reaction is highly endothermic and thus requires externalheating. A system incorporating only steam reforming will require alonger start-up time and a larger size for the micro-reactor 600 as wellas being less responsive to changes in feed composition.

Forced unsteady-state operation (FUSO) operation of the catalyticmicro-reactor 600, particularly auto-thermal reverse-flow operation,appears most promising from the viewpoints of productivity, energyefficiency and selectivity for reconciling the short-comings of eachprocess identified above. Further details are discussed by Matros, Y. S.“A Review: forced unsteady-state processes in heterogeneous catalyticreactors”, Canadian Journal of Chemical Engineering, 74, p. 566 (1996).Reverse-flow operation has the following beneficial properties: itreduces heat transfer resistance by minimizing the distance and timethat heat needs to travel within the micro-reactor (i.e., wherever heatis produced, it is consumed within a few milliseconds), therebysignificantly lowering average operating temperatures and heat losses(in turn requiring less insulation of the reactor skin). FUSOsubstantially eliminates the possibility of hot spots and thermalrunaway in the micro-reactor, and it results in reduced reactor size andpressure drop, which positively impact the cost and portability of thesystem. Reverse-flow operation in particular offers otherthermodynamically favorable conditions (i.e., declining temperatureprofile for exothermal reactions and increasing temperature profile forendothermic ones), which are impossible to attain during steady-state,unidirectional operation. Reverse flow operation additionally exhibitslow sensitivity to large composition and reagent flow rate fluctuations,resulting in easier operation and control of the micro-reactor.

The reverse-flow operation allows combined use of strongly exothermicpartial oxidation with endothermic, heterogeneous, steam reforming,followed by the slightly exothermic water-gas shift reaction. As anexample, in the case of methane, the following global reactions can beemployed by a reverse-flow micro-reactor 600:CH₄+2O₂→CO₂+2H₂O ΔH⁰ ₂₉₈=−802.0 kJ/molCH₄+H₂O

CO+3H₂ ΔH⁰ ₂₉₈=+206.1 kJ/molCO+H₂O

CO₂+H₂ ΔH⁰ ₂₉₈=−41.15 kJ/molwhere ΔH⁰ ₂₉₈ denotes the amount of energy in kilojoules/mol that isreleased (demonstrated by a negative number) or absorbed (demonstratedby a positive number) by each process.

By proper combination of the feed composition, a hybrid partialoxidation and steam-reforming process can be made slightly exothermic,it may be readily employed in the autothermal reverse-flow catalyticmicro-reactor. In order to initiate this hybrid process, themicro-reactor is first preheated to a uniform temperature required for“ignition” of the oxidation reaction. Thereafter, it continuouslyoperates via auto-thermal heat regeneration accomplished by periodicflow reversal. As an up-stream portion of the micro-reactor isprogressively cooled down by the cold reactants entering themicro-reactor, the methane partial oxidation process generates heat anda temperature wave propagates through the micro-reactor in the directionof the fuel flow. According to the present disclosure, the flow isreversed before the reaction zone reaches the exit of the micro-reactor.This causes the high temperature wave front to move in the oppositedirection. Thereby, the heat is effectively retained inside thecatalytic chamber of the micro-reactor. Furthermore, external reheatingis not needed to maintain the reaction operation, as shown by the graphsof temperature and reagent/product concentration profiles forreverse-flow, auto-thermal operation with methanol fuel in a forwarddirection (FIG. 14) and the reverse direction (FIG. 15). Fine-tuning ofthe temperature profiles in the micro-reactor can be accomplished byadjusting the steam reforming catalyst deposited therein.

In addition to favorable auto-thermal operation, the oscillatory motionof the high temperature wave in the micro-reactor leads to gasificationand removal of carbon and soot, which would otherwise be deposited onand poise the catalyst deposited on the channels of the micro-reactor600 due to endothermic methane cracking and exothermic Boudouardreactions. Furthermore, partial oxidation features much lower carbonmonoxide (CO) content in the product stream (as compared to steamreforming) which is an important consideration, since CO is adetrimental to the anode catalysts used in conventional proton exchangemembrane (PEM) fuel cells, such as may be used as fuel cell 800.

After several flow reversals, a quasi-steady state temperature profile,shown in FIG. 16, is established in the micro-reactor with an extendedzone of approximately uniform elevated temperature in the middle of thereactor (providing optimal conditions for the endothermic steamreforming reaction) and declining temperature profiles on both ends ofthe reactor (providing optimal conditions for the exothermic oxidationand water-gas shift reactions). This results in maximum reactorconversion and selectivity. The reverse-flow operation of themicro-reactor makes it possible to generate and control thespatio-temporal patterns of temperature, concentrations and catalyststates that cannot be attained under steady-state unidirectionaloperation. The heat is effectively retained inside the catalyticmicro-reactor, thus allowing for lower operating skin temperatures. Italso offers opportunities for exploiting catalyst dynamic properties,and results in much lower average operating temperatures, therebyreducing heat losses and pressure drop. Finally, as is also shown inFIG. 16, the temperature profiles dictate an optimal placement ofreforming catalysts (such as, but not limited to, nickel), oxidationcatalysts (such as, but not limited to, platinum (Pt) or rhodium (Rh)),and shift catalysts.

4.2 Microreactor Design

FIGS. 17 and 24 are schematic diagrams of one embodiment of a planarmicro-reactor 600. The micro-reactor 600 includes mixing chambers 400disposed in communication with one or more internal reactor channels606. The reactor channel(s) 606 have a catalyst deposition thereon forreacting with the mixed reagents received from the mixing chambers 400.The internal channels 606 further dispense the reaction products to thehydrogen separating membrane 700 due to the significant pressureresulting from the flow in microchannel geometry (inversely proportionalto the channel characteristic dimension in the forth power), as shown inthe insert of FIG. 24. The high temperature generated in the reactorchannels result in heating of the hydrogen separating membrane, furtherincreasing permeability and separation efficiency of the membrane.Further, the in-situ removal of hydrogen from the product stream withinthe reactor shifts the equilibrium of the reversible catalyticreactions, resulting in an additional increase in the overall hydrogenproduction.

The hydrogen separating membrane 700 may be used to selectivelytransport or filter hydrogen (or other desired compound) from thereaction products to a manifold, channel or container for collecting thefiltered products, including directly to anode of fuel cell mounteddirectly on top of the microreactor. In such an embodiment, the hydrogenseparating membrane 700 may be either Pd-alloy membrane or a mixed ionicelectronic conductor (MIEC) membrane, or other material of similargeneral functionality. The hydrogen separating membrane 700 may furtherbe integrated with a cover plate 630 for fitting the hydrogen separatingmembrane 700 in operative communication with the internal reactorchannels 606.

The planar design depicted in FIG. 17 offers the following positiveattributes. It readily allows for the stacking of similar parallel unitsfor increasing hydrogen production capabilities. There is a dramaticreduction in required ductwork or piping for fluid handling and asubstantial elimination of the need for auxiliary components forpumping, heating and the like, due to advantageous use of internalpressure drops and advantageous placement of its sub-components toprovide reheating from reaction products. In certain embodiments,though, some additional heating/insulation elements may be provided. Inaddition, highly reflecting scaffold (not shown), may likewise be usedto reduce radiation losses from the micro-reactor 600.

It is readily contemplated that the micro-reactor 600 may be operated byunidirectional flow and/or or reverse-flow mode. Experimentation hasrevealed that reactor selectivity in the range of 65-70% and conversionof 50-55% can be achieved for unidirectional auto-thermal conversion ofmethane to hydrogen with very little carbon monoxide (CO) content in theproduct stream. Further increase in the hydrogen yield could achievethrough recycling of the product stream. The auto-thermal state was verystable for many hours of continuous operation even when smallfluctuations were present in the reactor feed composition.

For reverse-flow auto-thermal operation, however, about a 7-10% increasein the hydrogen selectivity was achieved over unidirectional operation,while maintaining the same rate of conversion (Tables 1a and 1b).Experimentation has revealed that imposing flow reversal over thecatalyst also allowed for a decrease of the reaction ignitiontemperature by as much as 200° C., which is essential for the efficientstart-up of a cold micro-reactor 600. Both unidirectional andreverse-flow micro-reactors featured very low (about 30-40° C.) reactor“skin” temperature as long as they operated in an auto-thermal mode.This is because the distance and time that the heat produced fromexothermic oxidation reaction must travel before it is consumed by theendothermic steam reforming reaction is minimized, as both reactionsessentially occur on the same catalyst grain.

Unidirectional flow is readily accomplished with various existingchemical reactor designs. However, three alternative designs for thereverse-flow micro-reactors will now be disclosed.

Ideally, the valves of any reverse-flow micro-reactor 600 will exhibitthe following properties. The flow-reversal valves should have a deadvolume that is as small as possible (ideally zero), in order to minimizethe cross-talk between the un-reacted reagents and reaction productsimmediately after flow reversal. This will significantly reduce oreliminate the need to purge the micro-reactor 600. The valve(s) must beamenable for simple, low-power actuation to ensure overall systemeffectiveness and energy efficiency. The number of connecting pipes andmanifolds should be reduced to a minimum or, ideally, eliminated alltogether for minimizing cost of production and simplifying operation.Finally, the mixing chambers 400 and reaction channel 606 should beplaced in optimal locations to achieve additional functionality. Forexample, waste heat generated in the reaction channel 606 may be used topreheat the reagents in the mixing chamber 400, which results inincreased energy efficiency and enhanced reagent mixing of themicro-reactor 600.

EXAMPLE 1

FIG. 18 illustrates a rotating embodiment 401 of a reverse-flowmicro-reactor 600 that permits efficient integration (i.e., without anyadditional pipes or connecting manifolds) of a mixing chamber 400, azero-dead-volume rotating valve 404, and reaction channels 606. In thisembodiment, flow reversal is accomplished by rotating (using a motor orthe like) the mixing chamber 400 over a small angle about a fixedpivotal axis 404 (that may also be used for securing the micro-reactorassembly), thereby sequentially connecting the reaction channel 606 tothe left and right sides of the mixing chamber 400. The mixing chamber400 may be rotated between a first position 420 and a second position422. When in the first position 420, the reagent inlet ports 414 and theproduct exit port 416 are biased to allow flow of reagents in a firstdirection along the deposited catalysts in the reaction channels 606.Conversely, when in the first position 420, the reagent inlet ports 414and the product exit port 416 are biased to allow flow of reagents in asecond direction, substantially opposite to the first direction.

It should be noted that the reaction channels 606 may be placedsubstantially near, but not inside, the mixing chamber 400 to use theheat escaping the reaction chamber 702 in a radially outward directionfor reagent preheating in the mixing chamber 400. Although singlecircular channels are shown for the mixing chamber 400 and the reactionchannel 606 in FIG. 18, these channels can be made as complex as onewishes (e.g., a spiral or a Swiss roll configuration) to increasecontact area and/or reagent residence time.

Small leakages of reagents across the contact plane of the mixingchamber 400 and the reaction channel 606 may be used as a lubricationlayer for accommodating a smooth relative rotation of the two structurescontaining the mixing chamber 400 and the reaction channels 606. Ifneeded, the leaked reagents can be also collected and recycled back intothe reaction channels 606 to further increase the reaction yield.

Leakage can be further minimized by carefully polishing the contactsurfaces of the structures containing the mixer 400 and reactionchannels 606 or by using an intermediate thin separating plates (notshown) that may be firmly attached (cemented) to the mixer 400 and thereaction channels 606.

Further, the hydrogen separating membrane may be used to selectivelytransport or filter hydrogen (or other desired compound) from thereaction products to a manifold, channel, or container for collectingthe filtered products, including directly to anode of fuel cell mounteddirectly on top of the microreactor. The hydrogen separating membranemay further be integrated with the mixing chamber 400 for placing it inoperative communication with the internal reactor channels 606. In suchan embodiment, the hydrogen separating membrane may be a Pd-alloymembrane or a mixed ionic electronic conductor (MIEC) membrane, or othermaterial of similar general functionality, for example.

EXAMPLE 2

A third embodiment of the reverse-flow micro-reactor 600 is shown inFIG. 19. In this embodiment, flow reversal may also be accomplished bythe micro-reactor 600 in a valve-less fashion by placing one or moresuitable cover plates 482 and 484 having inlet ports 414 and outletports 416 for establishing the flow direction. The cover plates 482 and484 may internally incorporate the mixing chambers 480, which aredisposed to communicate with the reaction channels 606. In thisembodiment, the absence of valves, and thus of dead volume associatedwith valves, simplifies actuation by allowing use of a linear motor orany other actuator (not shown) to move the cover plates 482 relative tothe base plate 484 containing the reaction channels 606. The coverplates may be moved successively in a first position 482 and a secondposition 484, in order to bias reagent flow in a first or a seconddirection, respectively, along the reaction channels 606. Thisembodiment substantially eliminates reactor purging steps necessitatedby reagent-to-product cross-talk in existing systems. The bottom reactorplate with the catalyst channels and the top (valve) plate may befurther separated by an intermediate plate with appropriate openings andseal for leakage prevention

It should be noted the disclosed embodiments 1 and 2 for valves-lessreverse-flow operation can be used not only for hydrogen production, butalso for executing any other chemical reactions that can benefit fromthe reverse-flow operation (see, for example, Matros, Y. S, CatalyticProcesses Under Unsteady-State Conditions, Elsevier, N.Y., 1989 andMatros, Y. S. “A Review: forced unsteady-state processes inheterogeneous catalytic reactors”, Canadian Journal of ChemicalEngineering, 74, p. 566 (1996).). Also, the reverse-flow operation iscommonly used for reagent mixing and flow control in bioanalyticalinstruments (e.g., gas chromatography) and applications.

4.3 Examples: Experimental Results from Reactor Operation

Detailed experimental studies of both partial oxidation and steamreforming of methane and methanol fuels were undertaken for tubular androtating micro-reactor designs. Data was collected for reactorperformance (reaction conversion and selectivity towards hydrogen) for awide range of operating temperatures, total flow rate of reagents(expressed in terms of residence time or gas hourly space velocity(GHSV)), and feed compositions (e.g. methane-to-water ratios(CH₄:O₂:H₂O)). Further details on the experimental conditions are givenin Kikas, T., Bardenshteyn, I., Williamson, C., Ejimofor, C. Puri, P.,and Fedorov, A. G., Hydrogen production in the reverse-flow autothermalcatalytic microreactor, Proceedings of the Seventh InternationalConference on Microreaction Technology (IMRET), Lausanne, Switzerland(Sep. 7-10, 2003) and Kikas, T., Bardenshteyn, I., Williamson, C.,Ejimofor, C., Puri, P., and Fedorov, A., 2003, “Hydrogen Production inthe Reverse-Flow Autothermal Catalytic Microreactor: From Evidence ofPerformance Enhancement to Innovative Reactor Design”, Industrial &Engineering Chemistry Research, Vol. 42, pp. 6273-6279, which incorporated herein by reference.

The detailed studies of these micro-reactor processes clearlydemonstrate that by properly selecting the feed composition, the fulloxidation process could provide heat and water needed for thesteam-reforming process without requiring external heating. Some watermay be required to be added to the feed mixture for better temperaturecontrol of the micro-reactor 600, for example, by providing water forreaction quenching in order to avoid thermal runaway in themicro-reactor 600.

4.4 Fractal Deposition of Catalysts

Turning now to FIGS. 20-23, certain features of catalytic depositionalong the reaction channel(s) 606 of the micro-reactor 600 will now bedisclosed. A major factor in the cost of assembly of a micro-reactor 600is the cost of catalyst materials that need to be deposited on theinternal channels 606 to achieve desired level of chemical conversion.Previous attempts to reduce this cost have focused on replacing noblemetals such as Pt, Pd, Rh with cheaper alternatives, such as copper orother alloys featuring high catalytic activity. The catalyst load istypically uniformly deposited on an internal surface of the reactionchannels 606. However, the active surface and hence the catalyst loadingcan be drastically reduced, while the conversion rate remainsessentially unchanged, by using discontinuous catalyst depositionpattern such as fractals, for example, for spatial distribution of thecatalyst load. A significant reduction in cost of heterogeneousdiffusion-limited reaction micro-systems can thus be achieved throughsuch fractal deposition, due to lower required amounts of catalystdeposition.

The results are most significant for micro-reactors 600 in which theflow is characterized by a very low Peclet (Pe) number. For example, oursimulations indicate 75% reduction in the catalyst loading (cost) can beachieved with only 4% loss in the rate of chemical conversion as long asPe<1. These results counter intuition by indicating that the activesurface of heterogeneous, diffusion-limited reaction systems can bereduced significantly while maintaining essentially the same rate ofchemical conversion. Mathematically, this result owes to optimalplacement of singularities in the boundary conditions at points on thereaction channel(s) 606 where the active surface meets the inactivesurface. The local mass flux approaches asymptotically to infinity nearthese singularities, thereby establishing a mechanism for conserving thetotal rate of chemical conversion despite significant reduction in thecatalytically active surface area.

One process for achieving fractal distribution of a catalyst is nowdescribed, as an example only, as other distributions following thefundamental principles we established can be conceived. A given portionfrom the center of each segment of active surface is removed repeatedly,thus reducing the surface towards zero and consistently adding moresingularities into the boundary conditions. This configuration is knownas a Cantor set, the first four fractal iterations of which are shown inFIG. 23. As seen in FIG. 21, the iso-lines (lines of constantconcentration) are significantly disturbed (become spaced much denser)by the introduced singularities 760 very close to the active surface.This readily demonstrates the mass transfer enhancement when fractalstructuring of the active surface is used. Simulations indicate thatintroduction of periodic singularities into the boundary conditionsthrough fractal structuring of the active surface of the membrane 700allows a 76% reduction in the catalyst loading while losing only 2.25%of the original rate of chemical conversion, as demonstrated by thegraphs in FIG. 22, where the effect of the fractal-based reduction ofthe catalyst loading on the rate of mass transfer and its percentageloss are shown.

The effect of fractal structuring of the active surface is the mostprofound for micro- or nano-scale systems, for which the Peclet andRayleigh numbers are intrinsically small due to their smallcharacteristic length and scale. Similar results were obtained for themass transfer enhancement in the presence of natural convection, forwhich the relative importance of convection as compared to diffusion isgiven by the Rayleigh number (Ra). The total rate of mass transfer wasreduced by only 10% for a 76% drop in the active surface by fractalstructuring when Ra=1 (i.e., weak natural convection transport) and byabout 20% in the convection dominated situation characterized by thelarger Rayleigh number of 1000.

Further details are available in Phillips, C., Ben-Richou, A., Ambari,A., and Fedorov A., “Catalyst surface at a fractal of cost—A quest foroptimal catalyst loading”, Chem. Eng. Sci., 58 (11), 2403 (2003), whichis incorporated herein by reference.

4.5. Integrated System

FIGS. 24-28 show particular embodiments of an assembled integratedsystem 100 and its components. FIG. 24 is a side-view of a schematic ofthe integrated system 100 featuring a micro-reactor 600, an atomizer 500including an integrated piezo-electric device 510, and a membrane 700for hydrogen separation from the product stream. The pressure drop 660along the reaction channel(s) 606 is utilized for in-situ membraneseparation featuring locally optimal conditions for both hydrogenseparation (due to high temperature and pressure differential across themembrane) and hydrogen production (due to a shift in the reactionequilibrium towards more H₂).

FIG. 25 depicts a manufactured fluid reservoir 550 and a matrix ofejectors 535.

FIG. 26 depicts a scanning electron microscope (SEM) micrograph of thepyramidal structure of a matrix of ejectors 535 obtained via KOH-etch,with the nozzle 540 measuring approximately 15 μm. FIG. 27 depicts anSEM micrograph of a single ejector 535.

In addition, other embodiments can be envisioned based on variouscombination described system components (i.e., an atomizer, smartchannels, a membrane, and a microreactor). It includes, for example, anatomizer directly integrated with the hydrogen separating membrane (FIG.28), an atomizer integrated with the catalytic membrane reactor (FIG.29), an atomizer integrated directly with a fuel cell to supply fuel(FIG. 30), smart channels integrated with the fuel cell (FIG. 31), andsmart channels integrated with the hydrogen separating membrane (FIG.32).

4.6 Microreactor Manufacturing

Briefly, a procedure for fabricating the components of the micro-reactor600, as shown in FIG. 33 is as follows. Sequential photoresistspin-coating is performed on a silicon wafer. Lithography techniques arethen used to define the reaction channels 606 and the mixing chamber 400on the bottom plate 636 and connection through-holes on the top plate630. Cured photoresist is then removed using a developer solution (AZ400K Developer). ICP-etching of the exposed areas is then performed toproduce the reaction channels 606, the mixing chambers 400, andthrough-holes in the cover plate 630. The wafer is then cleaned usingacetone, and, finally, the reactor plates 630, 636 are cut to a propersize. The catalysts (Pt in this case) are then deposited on the interiorsurface of interior channels 606 using either a DC-sputterer or anelectron beam evaporator combined with shadow masking to depositcatalyst in predefined places only. A hydrogen separating membrane 700(for example, Pd-alloy) integrated with a porous silicon substrate mayreadily be used in the disclosed system 100.

Although the best methodologies of this disclosure have beenparticularly described in the foregoing disclosure, it is to beunderstood that such descriptions have been provided for purposes ofillustration only, and that other variations both in form and in detailcan be made thereupon by those skilled in the art without departing fromthe spirit and scope of the present invention, which is defined solelyby the appended claims.

1-75. (canceled)
 76. A method, comprising: providing an atomizer havingat least one ejector nozzle, at least one atomizer reservoir, and atleast one actuator, wherein the atomizer reservoir is disposed betweenthe ejector nozzle and the actuator; activating the actuator to generatean acoustical pressure wave for forcing the reactant through the ejectornozzle; and atomizing the reactant to produce an atomized reactant. 77.The method of claim 76, further comprising: mixing the atomized reactantwith a gas; transferring the atomized reactant/gas to a reactor, whereinthe reactor includes a membrane and a channel having a catalyst disposedthereon, and wherein the membrane bounds the channel on at least oneside; forming a fuel and reaction products by reacting the atomizedreactant/gas and catalyst in the channel; and separating the fuel fromthe atomized reactant/gas and reaction products using the membrane toproduce a substantially pure fuel steam.
 78. The method of claim 76,further comprising: collecting the fuel in a second channel of a fuelcell; and generating electricity from the fuel.
 79. The method of claim76, further comprising: focusing the acoustical pressure wave with astructure of the atomizer.
 80. The method of claim 76, furthercomprising: providing at least one channel that fluidically couples theatomizer and a reactant storage reservoir, wherein the channel includesa flexible membrane responsive to a signal to expand and contract across-sectional area of the channel; and transferring the reactant tothe atomizer from the storage reservoir by causing the flexible membraneto contract the cross-sectional area of the channel.
 81. The method ofclaim 77, further comprising: providing at least one channel thatfluidically couples the atomizer and the reactor, wherein the channelincludes a flexible membrane responsive to a signal to expand andcontract a cross-sectional area of the channel; and transferring thereactant to the reactor from the atomizer after atomizing the reactantby causing the flexible membrane to contract the cross-sectional area ofthe channel.
 82. The method of claim 77, further comprising: introducingthe atomized reactant/gas to the reactor in a first direction at a firstend of the reactor along the membrane; and introducing the atomizedreactant/gas to the reactor in a second direction at a second end of thereactor along the membrane, wherein introducing the atomizedreactant/gas in the first direction and the second direction isalternated to achieve a forced unsteady-state operation of the reactor.83-85. (canceled)